1. Technical Field of the Invention
This disclosure generally relates to an apparatus for manufacturing semiconductor wafers and, more particularly, to a polishing head of a chemical mechanical polishing apparatus.
2. Description of the Related Art
Chemical mechanical polishing (CMP) is a global technique for planarizing ultra large-scale integrated circuits (ULSI). By continuously feeding slurry onto an abrasive pad, the CMP is carried out to remove a step difference of layers formed on a wafer surface. Generally, the CMP is performed once or twice during processes such as, for example, depositing, exposing, and etching.
In CMP, the rear surface of a wafer is fixed by a polishing head and the front surface thereof is pressed against an abrasive pad installed on a turntable. The polishing head may be rotated to provide additional movement between the wafer and the turntable.
An effective CMP process requires that a wafer surface is uniformly planarized even at a high polishing speed. Characteristics such as a planarity of a wafer surface to be polished and a polishing speed are strongly affected by a relative speed between a wafer and an abrasive pad and a force pressing the wafer against the abrasive pad. The stronger the wafer pressing force is, the higher the polishing speed becomes. For this reason, in the event that a non-uniform force is applied by the polishing head to the wafer, the relatively strongly pressed portions of the wafer surface are polished faster than the relatively weakly pressed portions thereof.
In the CMP process, polishing uniformity is dependent on head configuration. Thus, membrane-type heads of an excellent polishing uniformity have actively been developed and applied by CMP apparatus companies. Furthermore, equipment for regulating the CMP characteristics of a wafer edge portion is needed with the presence of large-diameter wafers.
The configuration of a polishing head in a typical CMP apparatus is now described below with reference to FIG. 1.
Referring to FIG. 1, a polishing head 10 has a pressurizing structure in which only one pressure hole 20a is perpendicularly formed in a center support 20. When a gas pressure or a vacuum pressure is distributed throughout a membrane 23 via thru-holes 22a of a perforated plate 22, it is temporarily more lopsided toward an inner portion X1 of the membrane 123 than an edge portion X2 thereof, which is undesirable.
Due to the non-uniform pressurizing operation within a temporary time interval, the polishing uniformity is lowered and in particular a wafer edge slow phenomenon occurs. In other words, there is a slower polishing rate at the edge portion X2 of the wafer then at the inner portion X1.
The elasticity of the membrane 23 is also decreased by slurry. Specifically, the elasticity at the inner portion X1 is lowered more than that at the edge portion X2, which results in non-uniform expansion of the membrane. Due to the non-uniform expansion of the membrane, the wafer polishing uniformity is lowered and the wafer edge slow phenomenon occurs.
Embodiments of the invention address this and other limitations of the prior art.